Parsing Lisp S-Expressions with known schema in C#

Question

I'm working with a service that provides data as a Lisp-like S-Expression string. This data is arriving thick and fast, and I want to churn through it as quickly as possible, ideally directly on the byte stream (it's only single-byte characters) without any backtracking. These strings can be quite lengthy and I don't want the GC churn of allocating a string for the whole message.

My current implementation uses CoCo/R with a grammar, but it has a few problems. Due to the backtracking, it assigns the whole stream to a string. It's also a bit fiddly for users of my code to change if they have to. I'd rather have a pure C# solution. CoCo/R also does not allow for the reuse of parser/scanner objects, so I have to recreate them for each message.

Conceptually the data stream can be thought of as a sequence of S-Expressions:

(item 1 apple)(item 2 banana)(item 3 chainsaw)

Parsing this sequence would create three objects. The type of each object can be determined by the first value in the list, in the above case "item". The schema/grammar of the incoming stream is well known.

Before I start coding I'd like to know if there are libraries out there that do this already. I'm sure I'm not the first person to have this problem.

---

EDIT

Here's a little more detail on what I want as I think the original question may have been a little vague.

Given some SExpressions, such as:

(Hear 12.3 HelloWorld)
(HJ LAJ1 -0.42)
(FRP lf (pos 2.3 1.7 0.4))

I want a list of objects equivalent to this:

{
    new HearPerceptorState(12.3, "HelloWorld"),
    new HingeJointState("LAJ1", -0.42),
    new ForceResistancePerceptorState("lf", new Polar(2.3, 1.7, 0.4))
}

The actual data set I'm working on is a list of perceptors from a robot model in the RoboCup 3D simulated soccer league. I may potentially also need to deserialise another set of related data with a more complex structure.

Answer 1

I wrote an S-Expression parser in C# using OMeta#. It can parse the kind of S-Expressions that you are giving in your examples, you just need to add decimal numbers to the parser.

The code is available as SExpression.NET on github and a related article is available here. As an alternative I suggest to take a look at the YaYAML YAML parser for .NET also written using OMeta#.

Answer 2

Here's a relatively simple (and hopefully, easy to extend) solution:

public delegate object Acceptor(Token token, string match);

public class Symbol
{
    public Symbol(string id) { Id = id ?? Guid.NewGuid().ToString("P"); }
    public override string ToString() => Id;
    public string Id { get; private set; }
}

public class Token : Symbol
{
    internal Token(string id) : base(id) { }
    public Token(string pattern, Acceptor acceptor) : base(pattern) { Regex = new Regex(string.Format("^({0})", !string.IsNullOrEmpty(Pattern = pattern) ? Pattern : ".*"), RegexOptions.Compiled); ValueOf = acceptor; }
    public string Pattern { get; private set; }
    public Regex Regex { get; private set; }
    public Acceptor ValueOf { get; private set; }
}

public class SExpressionSyntax
{
    private readonly Token Space = Token("\\s+", Echo);
    private readonly Token Open = Token("\\(", Echo);
    private readonly Token Close = Token("\\)", Echo);
    private readonly Token Quote = Token("\\'", Echo);
    private Token comment;

    private static Exception Error(string message, params object[] arguments) => new Exception(string.Format(message, arguments));

    private static object Echo(Token token, string match) => new Token(token.Id);

    private static object Quoting(Token token, string match) => NewSymbol(token, match);

    private Tuple<Token, string, object> Read(ref string input)
    {
        if (!string.IsNullOrEmpty(input))
        {
            var found = null as Match;
            var sofar = input;
            var tuple = Lexicon.FirstOrDefault(current => (found = current.Item2.Regex.Match(sofar)).Success && (found.Length > 0));
            var token = tuple != null ? tuple.Item2 : null;
            var match = token != null ? found.Value : null;
            input = match != null ? input.Substring(match.Length) : input;
            return token != null ? Tuple.Create(token, match, token.ValueOf(token, match)) : null;
        }
        return null;
    }

    private Tuple<Token, string, object> Next(ref string input)
    {
        Tuple<Token, string, object> read;
        while (((read = Read(ref input)) != null) && ((read.Item1 == Comment) || (read.Item1 == Space))) ;
        return read;
    }

    public object Parse(ref string input, Tuple<Token, string, object> next)
    {
        var value = null as object;
        if (next != null)
        {
            var token = next.Item1;
            if (token == Open)
            {
                var list = new List<object>();
                while (((next = Next(ref input)) != null) && (next.Item1 != Close))
                {
                    list.Add(Parse(ref input, next));
                }
                if (next == null)
                {
                    throw Error("unexpected EOF");
                }
                value = list.ToArray();
            }
            else if (token == Quote)
            {
                var quote = next.Item3;
                next = Next(ref input);
                value = new[] { quote, Parse(ref input, next) };
            }
            else
            {
                value = next.Item3;
            }
        }
        else
        {
            throw Error("unexpected EOF");
        }
        return value;
    }

    protected Token TokenOf(Acceptor acceptor)
    {
        var found = Lexicon.FirstOrDefault(pair => pair.Item2.ValueOf == acceptor);
        var token = found != null ? found.Item2 : null;
        if ((token == null) && (acceptor != Commenting))
        {
            throw Error("missing required token definition: {0}", acceptor.Method.Name);
        }
        return token;
    }

    protected IList<Tuple<string, Token>> Lexicon { get; private set; }

    protected Token Comment { get { return comment = comment ?? TokenOf(Commenting); } }

    public static Token Token(string pattern, Acceptor acceptor) => new Token(pattern, acceptor);

    public static object Commenting(Token token, string match) => Echo(token, match);

    public static object NewSymbol(Token token, string match) => new Symbol(match);

    public static Symbol Symbol(object value) => value as Symbol;

    public static string Moniker(object value) => Symbol(value) != null ? Symbol(value).Id : null;

    public static string ToString(object value)
    {
        return
            value is object[] ?
            (
                ((object[])value).Length > 0 ?
                ((object[])value).Aggregate(new StringBuilder("("), (result, obj) => result.AppendFormat(" {0}", ToString(obj))).Append(" )").ToString()
                :
                "( )"
            )
            :
            (value != null ? (value is string ? string.Concat('"', (string)value, '"') : (value is bool ? value.ToString().ToLower() : value.ToString())).Replace("\\\r\n", "\r\n").Replace("\\\n", "\n").Replace("\\t", "\t").Replace("\\n", "\n").Replace("\\r", "\r").Replace("\\\"", "\"") : null) ?? "(null)";
    }

    public SExpressionSyntax()
    {
        Lexicon = new List<Tuple<string, Token>>();
        Include(Space, Open, Close, Quote);
    }

    public SExpressionSyntax Include(params Token[] tokens)
    {
        foreach (var token in tokens)
        {
            Lexicon.Add(new Tuple<string, Token>(token.Id, token));
        }
        return this;
    }

    public object Parse(string input)
    {
        var next = Next(ref input);
        var value = Parse(ref input, next);
        if ((next = Next(ref input)) != null)
        {
            throw Error("unexpected ", next.Item1);
        }
        return value;
    }
}

public class CustomSExpressionSyntax : SExpressionSyntax
{
    public CustomSExpressionSyntax()
        : base()
    {
        Include
        (
            // "//" comments
            Token("\\/\\/.*", SExpressionSyntax.Commenting),

            // Obvious
            Token("false", (token, match) => false),
            Token("true", (token, match) => true),
            Token("null", (token, match) => null),
            Token("\\-?[0-9]+\\.[0-9]+", (token, match) => double.Parse(match)),
            Token("\\-?[0-9]+", (token, match) => int.Parse(match)),

            // String literals
            Token("\\\"(\\\\\\n|\\\\t|\\\\n|\\\\r|\\\\\\\"|[^\\\"])*\\\"", (token, match) => match.Substring(1, match.Length - 2)),

            // Identifiers
            Token("[_A-Za-z][_0-9A-Za-z]*", NewSymbol)
        );
    }
}

public class Node { }

public class HearPerceptorState : Node
{
    public string Ident { get; set; }
    public double Value { get; set; }
}

public class HingeJointState : Node
{
    public string Ident { get; set; }
    public double Value { get; set; }
}

public class Polar : Tuple<double, double, double>
{
    public Polar(double a, double b, double c) : base(a, b, c) { }
}

public class ForceResistancePerceptorState : Node
{
    public string Ident { get; set; }
    public Polar Polar { get; set; }
}

public class Test
{
    public static void Main()
    {
        var input = @"
            (
                (Hear 12.3 HelloWorld)
                (HJ LAJ1 -0.42)
                (FRP lf (pos 2.3 1.7 0.4))
            )
        ";

        // visit DRY helpers
        Func<object, object[]> asRecord = value => (object[])value;
        Func<object, Symbol> symbol = value => SExpressionSyntax.Symbol(value);
        Func<object, string> identifier = value => symbol(value).Id;

        // the SExpr visit, proper
        Func<object[], Node[]> visitAll = null;
        Func<object[], Node> visitHear = null;
        Func<object[], Node> visitHJ = null;
        Func<object[], Node> visitFRP = null;

        visitAll =
            all =>
                all.
                Select
                (
                    item =>
                        symbol(asRecord(item)[0]).Id != "Hear" ?
                        (
                            symbol(asRecord(item)[0]).Id != "HJ" ?
                            visitFRP(asRecord(item))
                            :
                            visitHJ(asRecord(item))
                        )
                        :
                        visitHear(asRecord(item))
                ).
                ToArray();

        visitHear =
            item =>
                new HearPerceptorState { Value = (double)asRecord(item)[1], Ident = identifier(asRecord(item)[2]) };

        visitHJ =
            item =>
                new HingeJointState { Ident = identifier(asRecord(item)[1]), Value = (double)asRecord(item)[2] };

        visitFRP =
            item =>
                new ForceResistancePerceptorState
                {
                    Ident = identifier(asRecord(item)[1]),
                    Polar =
                        new Polar
                        (
                            (double)asRecord(asRecord(item)[2])[1],
                            (double)asRecord(asRecord(item)[2])[2],
                            (double)asRecord(asRecord(item)[2])[3]
                        )
                };

        var syntax = new CustomSExpressionSyntax();

        var sexpr = syntax.Parse(input);

        var nodes = visitAll(asRecord(sexpr));

        Console.WriteLine("SO_3051254");
        Console.WriteLine();
        Console.WriteLine(nodes.Length == 3);
        Console.WriteLine(nodes[0] is HearPerceptorState);
        Console.WriteLine(nodes[1] is HingeJointState);
        Console.WriteLine(nodes[2] is ForceResistancePerceptorState);
    }
}

Testable here:

https://repl.it/CnLC/1

'HTH,

Answer 3

Consider using Ragel. It's a state machine compiler and produces reasonably fast code.

It may not be apparent from the home page, but Ragel does have C# support. Here's a trivial example of how to use it in C#

Answer 4

Look at gplex and gppg.

Alternatively, you can trivially translate the S-expressions to XML and let .NET do the rest.

Answer 5

In my opinion a parse generator is unneccessary to parse simple S-expressions consisting only of lists, numbers and symbols. A hand-written recursive descent parser is probably simpler and at least as fast. The general pattern would look like this (in java, c# should be very similar):

Object readDatum(PushbackReader in) {
    int ch = in.read();
    return readDatum(in, ch);
}
Object readDatum(PushbackReader in, int ch) {
    if (ch == '(')) {
        return readList(in, ch);
    } else if (isNumber(ch)) {
        return readNumber(in, ch);
    } else if (isSymbolStart(ch)) {
        return readSymbol(in, ch);
    } else {
        error(ch);
    }
}
List readList(PushbackReader in, int lookAhead) {
    if (ch != '(') {
        error(ch);
    }
    List result = new List();
    while (true) {
        int ch = in.read();
        if (ch == ')') {
            break;
        } else if (isWhiteSpace(ch)) {
            skipWhiteSpace(in);
        } else {
            result.append(readDatum(in, ch);
        }
    }
    return result;
}
String readSymbol(PushbackReader in, int ch) {
    StringBuilder result = new StringBuilder();
    result.append((char)ch);
    while (true) {
       int ch2 = in.read();
       if (isSymbol(ch2)) {
           result.append((char)ch2);
       } else if (isWhiteSpace(ch2) || ch2 == ')') {
           in.unread(ch2);
           break;
       } else if (ch2 == -1) {
           break;
       } else {
           error(ch2);
       }
    }
    return result.toString();
}

Answer 6

Drew, perhaps you should add some context to the question, otherwise this answer will make no sense to other users, but try this:

CHARACTERS

    letter = 'A'..'Z' + 'a'..'z' .
    digit = "0123456789" .
    messageChar = '\u0020'..'\u007e' - ' ' - '(' - ')'  .

TOKENS

    double = ['-'] digit { digit } [ '.' digit { digit } ] .
    ident = letter { letter | digit | '_' } .
    message = messageChar { messageChar } CONTEXT (")") .

Oh, I have to point out that '\u0020' is the unicode SPACE, which you are subsequently removing with "- ' '". Oh, and you can use CONTEXT (')') if you don't need more than one character lookahead.

FWIW: CONTEXT does not consume the enclosed sequence, you must still consume it in your production.

EDIT:

Ok, this seems to work. Really, I mean it this time :)

CHARACTERS
    letter = 'A'..'Z' + 'a'..'z' .
    digit = "0123456789" .
//    messageChar = '\u0020'..'\u007e' - ' ' - '(' - ')'  .

TOKENS

    double = ['-'] digit { digit } [ '.' digit { digit } ] .
    ident = letter { letter | digit | '_' } .
//    message = letter { messageChar } CONTEXT (')') .

// MessageText<out string m> = message               (. m = t.val; .)
// .

HearExpr<out HeardMessage message> = (. TimeSpan time; Angle direction = Angle.NaN; string messageText; .)
    "(hear" 
        TimeSpan<out time>
        ( "self" | AngleInDegrees<out direction> )
// MessageText<out messageText>    // REMOVED    
{ ANY } (. messageText = t.val; .) // MOD
    ')' (. message = new HeardMessage(time, direction, new Message(messageText)); .)
    .